1. The Field of the Invention
The present invention relates to a method and apparatus for deliberately demagnetizing and remagnetizing a casing, situated in a borehole, with a known orientation to facilitate the subsequent orientation of tools passing therethrough.
2. The Prior Art
It is well known that when a hole is bored into the earth, using any known well-drilling method and apparatus, that the bore hole is not perfectly straight nor does it necessarily follow a regular geometric pattern, such as a helix or corkscrew path. Rather the drill bit follows a path generally of least resistance and thus waivers and deviates from a straight line path. Even with the various methods of controlling the drill bit, it is still impossible to drill a completely straight hole. Thus it becomes important to know the orientation of the hole during subsequent operations when, for example, it may be desirable to perforate a well casing in a certain direction.
Also there are some borehole seismic devices, both sources and detectors, which require orientation knowledge. For example, it is very useful to establish the orientation of a three-component geophone receiver deployed in a well in order to know which way the horizontal phones are directed. Magnetic compasses are often used for this purpose in uncased wells, but clearly this approach will not work once the well has been cased with steel pipe. Although gyrocompasses do work well inside steel casings, they are precision mechanical devices that are relatively expensive. The cost to run a gyrotool is around $3,500.00 per day. Since borehole seismic surveys may last from a few days to a week, it is desirable to have a lower cost alternative.
The gyrocompass tools are routinely run once in each well used in a borehole seismic survey in order to precisely survey the coordinates of the borehole as a function of depth. This is necessary because most well bores will not follow a linear path and the bottom of the well may be offset some distance from the surface location.